Annual balance measurements on the Lemon Creek Glacier, Alaska conducted
by the Juneau Icefield Research Program from 1953 to 1998 provide a continuous
46 year record.This is one of the
nine American glaciers selected in a global monitoring network during the IGY,
1957-58. These data have been acquired primarily by employing consistent field
methods, conducted on similar annual dates and calculated using a consistent
methodology.The record have been
until now precise, but of uncertain accuracy.Comparison of geodetic surface maps of
the glacier from 1957 and 1989 allow determination of glacier surface elevation
changes.Airborne surface profiling
in 1995, and comparative GPS leveling transects in 1996-1998 further update
surface elevation changes resulting from cumulative mass balance changes.Glacier mean thickness changes from
1957-1989, 1957-1995 and 1957-1998 were -13.2 m, -16.4 m, and –21.7 m
respectively.It is of interest
that the geodetic interpretations agree fairly well with the trend of sequential
balances from ground levelstratigraphic measurements.To date, however, the infrequent mapping methods in this study have
yielded specific balances averaging between 5 and 11% less that those resulting
from our annual on-site glacilogical modeling..For future studies this can be an
important factor.The grond data
are, therefore, the ones in which have the most confidence.These show cumulative ice losses of

–24.4 m (–22.0 m we) from 1957-1998.The
mean annual balance of the 46 record is

-0.48 m/a and a loss of at
least 24.7 m of ice thickness for the full 46 year period from 1953-1998.

To refine the
reliability of density determinations in this data set the effects of internal
accumulation from refrozen meltwater producing diagenetic ice structures in the
annual firnpack have been taken in to account.An unusual dearth of such structures
with the 1997-1998 firnpack provided a unique opportunity to facilitate
application for he providing technique over broad areas of the neve.This added top our ground truth andverified accuracy of the testpit
measurements used in these long-term mass balance computations.

This negative mass balance has fueled a terminal retreat
of 800 m during the 1953-1998 period.The annual balance trend indicates that despite a higher mean elevation
and a higher elevation terminus, from thinning and retreat, mean annual balance
has been strongly negative since 1977 (-0.78 m/a).Dramatically negative mass balances have
occurred in the 1990’s, with 1996 and 1997 being the only years with no
retained accumulation since field observations began in 1948.

Lemon Creek Glacier, Alaska was chosen as a representative glacier for
the 1958 IGY global glacier network (Figure 1).This choice was based on its
sub-arctic latitudelocation and on
the ongoing mass balance program of the Juneau Icefield Research Program (JIRP),
that had begun in 1948 (Miller, 1972, p. 4-6; Pelto and Miller, 1990, p.
121).JIRP has continued annual
balance measurements on Lemon Creek Glacier through 1998.Based on 1955-57 vertical aerial
photography a 1:10,000 scale map was produced in 1958 for the IGY (Heusser and
Marcus, 1964, Fig. 3).In 1957
Lemon Creek Glacier was 6.4 km long and had an area of 12.67 km2(Figure 2)(Heusser and Marcus, 1964, p.
63).In 1998 the glacier was 5.6 km
long and had an area of 11.8 km2 (Figure 3) (Marcus et al., 1995,
Fig. 6).From the head of the
glacier at 1300 m to the mean ELA at 1050-1100 m the glacier flows northward, in
the ablation zone the glacier turns westward terminating at 600 m.The glacier can be divided
into four sections: 1) Steep peripheral northern and western margins draining
into the main valley portion of the glacier. 2) A low slope ( 40)
upper accumulation zone from 1220 m to 1050 m.3) A steeper section (60) in
the ablation zone as the glacier turns west from 1050-850 m.4) An icefall (180) leading
to the two fingered termini at 600 m.The maximum thickness exceeding 200 m is 1 km above the icefall (Figure
4)(Thiel, 1957, p. 746; Miller, 1972, Fig. 74-77).

This paper will review the compilation of the 46 year mass balance record
for this glacier (1953-1998).Only cursory mass balance observations were made from 1948-1952.On Lemon Creek Glacier no early ablation
season or late accumulation season measurements are made, thus, summer and
winter balance cannot be separated.The only previous publication of annual balance are for the 1954-1958
period (Heusser and Marcus, 1964; p. 69).The annual balance record has been verified using geodetic methods based
on a comparison of the1:10,000 scale maps made of the glacier in 1957 and in
1989 (Marcus and others, 1995: p. 155).Further surface radar altimetry using airborne surface profiling has
documented the geodetic mass balance change from 1989 to 1995 (Sapiano et al;
1998, Table 5).In 1996-1998
comparative GPS surface profiling has further updated the surface elevation
change.Without these geodetic
measurements the error of annual balance measurement using stratigraphic methods
is difficult to assess, especially in view of the dearth of detailed terminus
area ablation measurements.

Each year the mass balance of Lemon Creek Glacier has been determined
using stratigraphic field measurements of varying distribution and number.At a minimum each year the field studies
comprise repeated transient snowline mapping in early July and the ELA in early
September, and net retained accumulation measurements at four mass balance
control sites (Figure 5).

Each year JIRP completes between July 10 and July 20 a series of testpits
in which the thickness and density of the previous winters retained accumulation
is measured.A continuous density
profile from the surface to the base of the annual firnpack provides a direct
measure of the annual water equivalent thickness (LaChapelle, 1954, p.
19-21).The base of the annual
accumulation increment is identified by a continuous dusty horizon formed on the
previous summer surface.This layer
is well developed during each ablation season on the Lemon Creek Glacier
(LaChapelle, 1954, p. 25).LaChapelle (1954, p.24) noted that one of the first characteristics which
becomes apparent upon examination of the testpit profiles is the remarkable
uniformity of firn density in vertical profile, and in distribution over the
glacier, and with time during the summer.The average bulk density is
determined each year, and has ranged from 0.54-0.58 Mg/m3, including
diagenetic ice (0.54-0.55 Mg/m3in 1998).

In addition to the four
standard sites, net annual retained accumulation thickness has been observed at
up to 300 points in a single summer season.These
additional measurements above the transient snowline have been rammsonde
profiles, and by crevasse stratigraphy in transects starting near the standard
testpit sites or the transient snowline.

Ablation stakes, driven into the firn in the accumulation zone record the
ablation of the remaining firnpack in the accumulation zone between the testpit
accumulation measurements in July and the end of the ablation season inearly September.This provides an essential measure to
adjust the July accumulation thickness testpit measurements to the end of the
ablation season. The maximum number of such ablation stakes used during a single
season was 200 in 1967.During the
several years where more than 30 ablation stakes were emplaced, it is apparent
that ablation rates above 900 m are nearly constant on the Lemon Creek Glacier.
Below 900 m ablation rates increases with decreasing surface elevation. Table 1
includes the mean ablation rate determined from stake ablation at locations used
during different ablation seasons, in particular this table illustrates the
similarity of ablation above the icefall at900 m from July through early
September.

The transient snowline position is mapped in early July and the ELA in
early September to provide a further check on the amount of ablation. The ELA
provides a crucial tie point for the glacier's annual balance
determination.The calculation of
annual balance for each year depends on extrapolations from the standard testpit
sites and ELA, based on the detailed mass balance measurements made across the
glacier from 1954-1957, 1965-1969, 1972-1975, 1982-1984, and 1998.Except for the years from
1954-1957 with mass balance calculated by Heusser and Marcus (1964, p. 69),
annual balance is determined using data from the same five data points, four
testpits and the ELA.

JIRP has used consistent field methods on similar dates to provide
comparable records for each year.Meier and Walters (1989, p.367) noted that glacier mass balance can be
determined using area-weighted data from a single observation point. This can be
done only after the weights for data points have been determined from in-depth
multi-year mass balance studies, and calculations made of the representative
area for each point.This method
using typically 3-5 standard measurement points is employedby the USGS on their benchmark glaciers
Gulkana, Wolverine and South Cascade Glacier (Krimmel, 1994, Fig. 1; Mayo and
Trabant, 1992, Fig. 1; March and Trabant, 1995, Fig. 1).We have applied a comparable
density of measurements on Lemon Creek Glacier.

A detailed map of the accumulation pattern on the Lemon Creek Glacier has
been made from accumulation measurements made at several hundred locations
during some summers both in July and August.Table 2 indicates that indeed the
pattern is consistent and provides an accurate assessment of mass balance.In 1998, 300 accumulation measurements
were made to supplement the control record from the standard testpits.The pattern of mass balance remained the
same despite glacier thinning and the notably negative balance to the pattern
observed in 1954, 1960, 1967, 1972, 1978 and 1984 (Figure 6).

Ablation measurements in the ablation zone (region below
the July transient snowline) are the largest source of error.Although extensive ablation stake arrays
have been used in some years, this has not been possible in others. Based on 11
years of extensive ablation zone ablation measurements the distribution of
ablation in years of incomplete records for specific July 1 transient snowlines
and September ELA’s has been estimated.It has been observed that seasonal ablation above the transientsnowline is nearly uniform, while
ablation in the ablation zone is not.The July 1 transient snowline sector best represents early season
ablation and the change in elevation to the early September ELA sector is the
best measure of ablation for the remainder of the ablation season.Total annual ablation is calculated from the summer season rise in the
transient snowline between early July and September>This is based on the observed firnpack
water equivalence in early July in the zone where the ELA is later observed to
be in September.The
specific ablation between July and early September at that location, thus
references the entire ablation zone, using measured summer ablation data from
the years listed in Table 1.This
method assumes the pattern of ablation across the glacier in the ablation zone
to be similar from year to year, and that the relationship to ablation in the
accumulation zone is consistent.The data in Table 1 supports this consistency in ablation in the
accumulation zone4.

The relationship between net accumulation zone ablation
and ablation zone ablation has also been observed during at least 7 summer
seasons.This points at
accumulation zone ablation for the summer months as an additional indicator of
ablation zone ablation.

Values of mass balance recorded at the four principal testpit sites and
the change in transient snowline are weighted for the respective altitude zone
of the glacier that each represents.The mean value for each elevation zone and each site value has been
determined.The site value divided
by the mean value yields the weight for each site.These values are shown in Table 2.Mass balance control site A and B
are used to determine the mean annual accumulation for the upper section of the
glacier above 1175 m.Site C
represents the mean accumulation area for the glacier from 1100-1175 m, Site D
represents the mean for the elevation zone from 1025-1100 m.The transient snowline altitude change
from early July to September is used to determine the mean ablation rate for the
ablation zone.It is
axiomatic that the mass balance is then the sum of the products of the area and
mean annual balance of each area-elevation zone.

The surface area for each elevation band has been determined from the
1:10,000 surface map of 1957, with the same areas applied up to 1977.From 1977 to 1998 the surface
areas of the 1989 topographic map has been used.The changes in area of all but the
lowest elevation band were less than 1%.Terminus area changes mapped in 1965 have been used for the lower
elevation band area for the 1965-1977 period.

MASS BALANCE RECORD

The mass balance record determined solely from field measurements (Table
3), yields an overall mass balance of –22.1 m from 1953-1998.This represents a significant thinning
of the glacier over the 46 year interval.With an estimated volume of 901.7 million m3, the loss
determined up to 1989 of -131.9 million m3 was 15% of the total
glacier volume (Marcus and others, 1995: p. 159).This geodetic determination of long-term
1954-1989 mass balance change, represented a 13.2 m change in mean glacier
thickness.Airborne surface
profiling (Sapiano et al., 1998: Table 5) noted an additional loss of 3.2 m
ofglacier surface elevation from
1989 to 1995.Comparative GPS
surface profiling in 1996-1998 indicated an additional

–
4.6 m of glacier surface change.The annual balance record of -12.7 m we (13.9 m of ice thickness) from
1957-1989 compares well to the thinning identified from geodetic methods of
1957-1989 of -13.2(Marcus and
others, 1995, p. 159).The annual
balance record of –17.1 m we (-19.0 m of ice thickness) from 1957-1995 also
matches wellthe airborne surface
profiling 1957-1995 change of -16.4 m (Sapiano et al, 1998, Table 5 and Fig.
2).This flight was made in
late June when ice thickness was 1-2 m greater on average than at the end of the
1995 field season.The error in
both geodetic programs is less than 1.5 m.Table 4 illustrates overall
glacier mass balance changes determined by the three different means: airborne
surface profiling, surface mapping, and field measurements.

The annual balance record indicates that from 1957-1976 mass balance loss
was –0.23 m/a and thinning was modest on the upper reaches of the glacier.Despite a higher mean elevation and a
higher terminus elevation due to glacier retreat, mean annual balance has been
increasingly negative since 1977, averaging -0.78 m/a.The record is particularly negative
since 1990, -1.04 m/a (Figure 6).In 1996 and 1997, no accumulation was retained on Lemon Creek Glacier for
the first time since observations were initiated in 1948.In 1998, a small patch of accumulation
was retained on the upper part of Observation Peak at the head of the glacier
basin, covering less than 1% of the entire accumulation area. The ELA has risen
above the glacier’s neve each of the last three years.

Annual Balance Errors

JIRP recognizes that Lemon Creek Glacier annual balance measurement
program has gaps in its record due to logistical and time restrictions that lead
to some interpretation errors.The
ongoing goal is to minimize such and to continue to enhance the record
reliability.For example, potential
errors in accumulation measurement occur when meltwater percolates through the
current annual firn increment and refreezes within a lower firn increment.This internal accumulation
(Trabant and Mayo, 1985, p. 114), is not measured in most mass balance
programs.Bazhev (1986, p. 168-169)
noted that while most meltwater that refreezes does so in the current annual
increment some does percolate beyond into previous firn increments.This recaptured meltwater occurs
as meltwater refrozen in diagenetic ice structures.This ice and all internal accumulation
develops prior to the snowpack becoming totally isothermal at
0oC.On
Lemon Creek glacier this occurs prior to July 1 (Miller, 1972, p. 35; Miller,
1955, p.292).Thus, internal
accumulation does not occur after July 1 and ablation measurements are
representative of summer ablation losses.

To determine how much meltwater is retained as diagenetic structures the
walls of the testpit are surveyed.Crevasse stratigraphy has been useful source of this information in 1982
and 1984.The total water
equivalent of the three most recent annual firn layers are checked in early
July, early August and the following July.In each of these years the 1 and 2 year old increments experienced a net
gain of 1-2% water equivalent, suggesting minor internal accumulation that is
not accounted for when sampling only the most recent accumulation layer. This
procedure can be repeated in the future for more accurate measurement of this
increase.In most cases internal
accumulation below the most recent annual increment is viewed as
negligible.

Another potential error involves the assessment of ablation in the
ablation zone, whichis based on
correlation with ablation measured at the transient snowline (Table 2).Errors may be expected in years in which
the ablation pattern is atypical.The relationship of transient snowline changes to observed surface
ablation above the ELA is the only potential check on this, albeit an indirect
measure.

On Lemon Creek Glacier, ablation during several summers in the
accumulation zone has been noted at extensive arrays of stakes during several
summers.The standard deviation in
surface ablation was found to be less than 0.05m/month, which gives support to
the use of ablation stakes and the ELA to adjust the July firn depth.

The most serious error to be considered derive from completing
annualbalance measurements
considerably before the end of the true ablation season, when possible annual
balance has been observed and recorded at the end of the ablation season in
several years.Ablation is severely
reduced after August 25, thus, when theELA measurement can be made early September it provides an acceptable
measure of the final position.

CONCLUSION

The annual balance record of the Lemon Creek Glacier extends over 46
years showing an increasingly negative trend in annual balance.The total loss is substantial with a net
thinning of 24.4 m.This record has
been obtained from a consistent set of yearly measurements using comparable
methods and at similar annual dates.The result is a record in which we have confidence, though it is not
precise.Moreover, comparison with
long term geodetic mass balance determinations from two different measurement
methods demonstrate, that this annual balance record is acceptably
reliable.Specific winter and
summer balances of course cannot be differentiated from this record.

The lesson of this documentation is that mass balance measurements with a
limitedannual density of coverage
can still be reliable if there is: 1)An independent check using surface remapping, airborne profiling and/or
GPS surface leveling: 2) A program of extensive supplementary measurements
during at least some years to refine mass balance patterns and likely
errors.

Identification of errors and the refinement of mass balance patterns are
an ongoing process that can be used to reassess mass balance records.In 1998 JIRP completed a more detailed
mass balance assessment of Lemon Creek Glacier examining accumulation at more
than 300 locations.This data set
helps to continue refinement ofthe
mass balance pattern.Also of
importance in 1998 was the lack of development of diagenetic ice for the second
consecutive year.This is an
indication that the spring snowpack was not cold enough to lead to substantial
refreezing of meltwater.This is
important information regarding the 1998 mass balance, because 7-11% of the
retained annual accumulation is typically diagenetic ice.Failure to retain this meltwater
increases net ablation.The reason
for the warmer snowpack was that winter temperatures were +4oC above
the long term mean at the Juneau Airport seven miles from the glacier at sea
level.At Camp 17 at 1300 m on the
edge of the Lemon Creek Glacier's neve, mean March and April, 1998 temperatures
averaged -5oC, to warm to keep the snowpack below
0oC.The result of the
failure to recapture meltwater as internal accumulation is increased ablation
and negative annual balances on Lemon Creek Glacier.

ACKNOWLEDGEMENTS

The support of NSF, NASA, U.S. Army Research Office, the
Murdock Charitable Trust and University of Idaho have been essential to this
project, as well as the Foundation for Glacier and Environmental Research,
Pacific Science Center, Seattle, WA.

Year

800 m

1050 m

1175 m

1955

.060 m/day

.051 m/day

.050 m/day

1956

.068

.048

.046

1957

.062

.046

.045

1958

.052

.042

.042

1966

.055

.043

0.52

1967

.060

.056

.058

1968

.058

.052

.050

1984

.060

.053

.054

Table 1.Mean daily ablation over a minimum 36
days during eight different field seasons.The mean daily ablation for each elevation is the mean of at least three
separate measurement sites.

Year

Site A

Site B

Mean

Site C

Mean

Site D

Mean

1955

3.13

3.22

3.28

2.87

2.90

2.10

1.95

1957

2.75

2.81

2.88

2.60

2.73

1.78

1.60

1962

2.80

2.75

2.86

2.45

2.52

1.21

1.14

1965

3.02

3.10

3.14

2.78

2.74

1.72

1.50

1966

2.48

2.59

2.57

2.17

2.28

1.36

1.41

1967

2.53

2.44

2.52

2.25

2.19

1.18

1.05

1969

2.90

2.87

3.01

2.48

2.63

1.66

1.61

1972

2.38

2.36

2.49

2.15

2.15

1.10

1.02

1982

2.45

2.41

2.55

2.20

2.32

1.38

1.15

1984

2.68

2.71

2.78

2.48

2.59

1.65

1.52

Table 2.Comparison between Contol Site
accumulation layer thickness and mean accumulation layer thickness at 8 other
sites within the elevation band used for that Control Site.Each of Control Site A-D have been
checked every year. Site A and B1175+, Site C 1100=1175, Site D 1025-1100.